The present invention relates to a differential pressure transmitter which can measure a pressure difference between two points as a process variate and simultaneously measure a high or low static pressure.
In order to measure, e.g., a flow rate of a fluid in a pipe, an orifice plate is disposed in the pipe to produce a fluid resistance, and a pressure difference between the upstream and downstream sides of the orifice plate is measured to calculate the flow rate on the basis of a predetermined calculation. A differential pressure transmitter used for differential pressure measurement of this type is arranged such that the respective measurement pressures are supplied to high- and low-pressure barrier diaphragms, and movement of a sealed liquid caused by the pressure difference at that time is extracted as an electrical output in accordance with strain of a pressure sensor disposed by partitioning a sealing circuit. In addition, a differential pressure transmitter of this type is required to measure a small differential pressure under a high static pressure (e.g., plant reaction, boiler internal, and water supply pressures). However, a differential pressure measuring pressure sensor is generally affected by a static pressure and hence cannot perform differential pressure measurement with high accuracy under the high static pressure. For this reason, in a known apparatus, a static pressure measuring pressure sensor is incorporated in a differential pressure transmitter used under the high static pressure so that a zero-point change caused by a static pressure change is compensated. An example is a "differential pressure detector" disclosed in Japanese Patent Laid-Open (Kokai) No. 54-67480.
In this detector, as shown in FIG. 4, two strain resistance elements 3a and 3b of a static pressure measuring pressure sensitive element (sensor) 3 are respectively connected to a bridge circuit 1 and an operational amplifier 2 to control inputs thereof. The resistance element 3a determines a resistance change caused by a static pressure change so as to compensate for a zero-point change caused by the static pressure change, and the other resistance element 3b determines a resistance change caused by the static pressure change so as to compensate for a span change caused by the static pressure change and controls the input of the operational amplifier 2, thereby measuring a differential pressure with high accuracy. Note that in FIG. 4, reference symbols A and B denote differential pressure output terminals, and reference numeral 4 denotes a differential pressure detecting pressure sensitive element (sensor); 4a and 4b. strain resistance elements of the sensor 4; 5, a fixed resistor; 6, a power source; 7, a fixed resistor; and 8, a voltage/current converter.
However, according to either of the conventional differential pressure transmitter incorporating a static pressure measuring pressure sensor therein and the above-mentioned differential pressure detector (Japanese Patent Laid-Open (Kokai) No. 54-67480), a differential pressure measuring pressure sensor and a static pressure measuring pressure sensor are incorporated in a body having a barrier diaphragm, resulting in a complex body structure, cumbersome manufacture and assembly, and high cost. In addition, since a structure of a body incorporating a static pressure measuring pressure sensor and that of a body not incorporating it are apparently different from each other, two types of body must be manufactured, so that types of body are increased and hence parts control becomes troublesome. Furthermore, even when only one of the sensors malfunctions, the entire body need be replaced, resulting in an economical disadvantage.